I dont believable servo motors work like that (with steps), in general.
A stepper motor is part of an Open Loop system, you tell the stepper motor to "move" x-amount of steps and then it will stop.
A servo motor is part of a Closed Loop system, you tell the servo to move (and continues to spin) and an encoder reads its movement and sends that info to "something" to work with the info.
More then likely it would work simulare to your last sentence.
When the M6 command (toolchange) is read, the spindle will stop (where ever it wants to stop), then it will slowly spin in-till an encoder (or proxy switch) reads/finds correct point/orientation, then stop again and do a tool change.
Googling
Mach3 ATC might help ya out (ATC = Automatic Tool Changer).
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